1. Field of the Invention
The present invention relates to a hub and, more particularly, to a hub for a bicycle.
2. Description of the Related Art
A conventional ratchet hub for a bicycle in accordance with the prior art shown in FIGS. 6 and 7 comprises a shaft 10, a hub body 11 rotatably mounted on the shaft 10 and having an inside formed with a fixing recess 110, a locking ring 111 secured in the fixing recess 110 of the hub body 11 to drive and rotate the hub body 11 and having an inside formed with a plurality of oneway ratchet teeth 112, a drive seat 12 rotatably mounted on the shaft 10 and having an end face 120 extended into the locking ring 111 and formed with a plurality of receiving slots 122, a plurality of pawl members (not shown) each pivotally mounted in the respective receiving slot 122 of the drive seat 12 and each meshing with the oneway ratchet teeth 112 of the locking ring 111, and a plurality of ball bearings 14 mounted between the drive seat 12 and the shaft 10.
In assembly, the hub body 11 is connected to a wheel (not shown) of the bicycle, the shaft 10 is connected to a frame (not shown) of the bicycle, the drive seat 12 is connected to a freewheel 13 of the bicycle, and the freewheel 13 of the bicycle is driven by a chain (not shown) which is driven by a chainwheel (not shown) which is driven by a pedal (not shown) that is pedalled by a rider.
In operation, when the freewheel 13 is rotated normally, the drive seat 12 is rotated with the freewheel 13 to move the pawl members. At this time, the pawl members are locked in the oneway ratchet teeth 112 of the locking ring 111, so that the locking ring 111 is rotated by the drive seat 12 to rotate the hub body 11 which rotates the wheel. On the contrary, when the freewheel 13 stops rotating or when the rotation speed of the wheel is greater than that of the freewheel 13 (when the bicycle is moved on the downward slope), the rotation speed of the locking ring 111 is greater than that of the drive seat 12, so that the oneway ratchet teeth 112 of the locking ring 111 press each of the pawl members. Thus, the drive seat 12 performs an idle rotation, so that the locking ring 111 and the hub body 11 are not rotated by the drive seat 12 and will not interfere with rotation of the drive seat 12. When the rotation speed of the drive seat 12 (or the freewheel 13) is greater than that of the hub body 11 (or the wheel), the locking ring 111 and the hub body 11 are rotated by the drive seat 12 again.
However, when an external force is applied on the hub due to a violent hit or jump, the external force applied on the hub is entirely supported by the ball bearings 14 as shown in FIG. 7, so that the ball bearings 14 are easily distorted or deformed due to a stress concentration, thereby decreasing the lifetime of the ball bearings 14. In addition, when the ball bearings 14 are worn out or even broken, the shaft 10 is easily damaged by the broken pieces of the ball bearings 14, thereby decreasing the lifetime of the shaft 10. Further, each of the ball bearings 14 has a larger size, so that the thickness of the hub cannot be changed freely according to the practical requirement, thereby decreasing the versatility of the hub.